[Matrix]: fix a bug in get_portion in matrix_store.
[Matrix]: fix a bug in get_portion in matrix_store.

const get_portion and get_portion may do very different things.

flashxio/FlashX,zheng-da/FlashX,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,flashxio/FlashX,icoming/FlashGraph,icoming/FlashX,zheng-da/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,icoming/FlashGraph,icoming/FlashX,zheng-da/FlashX,icoming/FlashX,flashxio/FlashX,icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX